This invention relates to gearsets, and more particularly, to helical gearsets. More specifically, the gearsets disclosed herein are single helical gearsets utilizing steel gear blanks, in which the gear teeth are hobbed or otherwise formed and then are carburized (or otherwise hardened) and ground (if necessary) to an acceptable American Gear Manufacturers Association (AGMA) quality and reliability level, and in which the gearsets are AGMA rated.
Generally, a gearset comprises a pinion in mesh with a gear, the smaller being the pinion and the larger being the gear. Depending on whether the pinion or the gear is the driven member, the gearset may be a speed reducer or a speed increaser. In this disclosure, the term "speed reducer" will be utilized. However, within the broader aspects of this invention, it will be recognized that the gearsets of the present invention may be utilized with speed increasers as well as with speed reducers.
The gear ratio is the number of teeth on the gear divided by the number of teeth on the pinion. Helical gears have teeth which spiral around the body of the gear, and the helix angle is the inclination of the gear tooth in lengthwise direction with respect to the axis of rotation of the pinion or the gear. Helical gearsets generally have two or more teeth in contact with one another at any given time (referred to as overlap), and the contact begins at one end of the tooth and extends along a diagonal line across the width of the tooth. Generally, helical gears are favored over spur gears because they can carry higher loads at higher speeds, and are generally of smoother and quieter operation. However, it will be understood that spur gears are merely one embodiment of helical gears in which the helix angle is 0 degrees. Thus, the gearsets of the present invention apply to spur gears as well as to helical gears.
Gear designs have been an evolving art over many centuries. As more information has become known about mathematics, gear geometry, kinematics, strength of material, fabrication, and lubrication, much of the complicated gear design information and operational knowledge has been condensed into more manageable formulas or "rules of thumb", which allow the gear designer to design and manufacture gearsets having desired operational characteristics and load-carrying capabilities. Over the years, the American Gear Manufacturers Association (AGMA), of Arlington, Va., has developed and published a series of standards and ratings which enable gear designers and users to make rating calculations to establish that a given gearset is suitable in size and quality to meet the specified requirements of a gear application (i.e., to ensure that the gearset will transmit a specified horsepower at a known speed under a desired loading condition for a predetermined service life). These AGMA standards and ratings are updated periodically, and thus are an accurate reflection of the current state of the art in gear design and manufacture. In many gear applications, there is a contractual obligation that the gearing must meet all applicable AGMA standards.
In the design of helical involute gear teeth, AGMA standards require that the gear tooth design have sufficient bending strength and pitting resistance to result in a gearset which will carry its intended horsepower load for a prescribed service life with a desired level of reliability. Generally, the bending strength of gear teeth is a fatigue-related phenomena, dependent on the resistance to cracking of the gear tooth at the tooth root fillet caused by repeated application of bending loads to each tooth each time the gear tooth is in mesh with its mating gear. Pitting resistance is also a fatigue-related phenomenon. However, pitting is a result of contact pressures (i.e., Hertz stresses) between the meshing gear teeth of the gearset exceeding the limitations of the gear tooth material. In rating a gearset, it is necessary to rate the gearset both with regard to its resistance to pitting at a rated load (typically expressed in horsepower), and with regard to bending strength at its rated load or horsepower. The lower of the pitting resistance power rating or the bending strength power rating is then used as the power rating for that gearset. AGMA Standard 218.01, dated December, 1982, sets forth the pitting resistance and bending strength of spur and helical involute gear teeth. AGMA Standard 218.01 is herein incorporated by reference.
Over the years prior to this invention, gearset designs have evolved which work well for their intended uses or applications. As advances in knowledge came along, the AGMA standards were updated so that the AGMA standards reflected both the state of the art and operational experience of many gear manufacturers or users of gears made in accordance with AGMA standards. However, in general, the AGMA standards or ratings did not take the cost of manufacturing a gearset into account. Instead, a series of general design guidelines or rules of thumb of helical gear design evolved. First, it was generally recognized that the ratio of pinion face width divided by pinion pitch diameter should be 1.5 or less, so as to avoid torsional twist of the pinion which tends to concentrate the load on one end of the pinion. Additionally, except for extremely low speed (e.g., about 50 rpm or less) applications, the minimum number of pinion teeth should be 14 or more. However, for most applications, 16 or more pinion teeth are preferred. A fewer number of teeth will result in unsatisfactory wear and undue noise. In helical gearsets, the face overlap (overlap) should be 1.2 or greater so as to ensure that, at all times, there is load sharing by adjacent teeth. Additionally, it is usually advisable to keep the helix angle relatively low (e.g., 5-15 degrees) so as to limit the thrust loads applied to the helical gear.
As noted above, cost considerations are not taken into account by the AGMA standards and rating system, and are not reflected by the above-noted generally followed design guidelines. However, gear designers have developed some costing rules of thumb which are usually taken into account during the design of a gearset. Generally, it was heretofore thought that a helical gearset having the shortest center distances and the widest faces practical are most cost-effective. Also, coarse pitches were thought to be more costly than fine pitches. As will be noted, these cost guidelines are at direct odds with the above-noted design guidelines in many respects.
Over the years, helical gearset designs have evolved which work well and meet the AGMA standards and rating system. However, the question remained: Were the prior art gearsets the most economical designs which met the AGMA rating standards?